"This puzzle – these unexpected, very
high-energy gamma-ray emissions – come not from the pulsar,
but from the wind," said study leader Felix Aharonian of the
Dublin Institute for Advanced Studies in Ireland. "We are
convinced that this is the most natural way to explain this path
of emission."

The detailed findings of the study were published online today
(Feb. 15) in the journal Nature.

The Crab nebula is one of the most studied objects in space. It
is the wreckage of a violent stellar explosion, called a
supernova. The dying star was located 6,500 light-years from
Earth in the constellation Taurus when the spectacular supernova
explosion occurred. The light from this blast was noticed and
recorded in the year 1054 by Chinese and Native American
skygazers.

At the heart of the Crab nebula is a pulsar – the remains of the
original star's core that collapsed in on itself into a
super-dense, spinning neutron star. This pulsar, which is so
dense that it has a greater mass than the sun, spins 30 times a
second. [ 50
Fabulous Deep-Space Nebula Photos ]

From within the nebula's colorful folds of gas, the Crab pulsar
emits a
continuous beam of radiation that sweeps around, similar to a
lighthouse. When viewed through ground-based telescopes on Earth,
these beams appear to be pulsed.

When gamma-ray beams were detected at energy levels higher than
any theories predicted, scientists' initial thoughts were
that the emissions were being produced by the pulsar, since the
radiation also appeared to pulse.

But by tracing the path of the energy, Aharonian and his
colleagues predict that the gamma-rays are produced instead by an
accelerating wind that originates near the pulsar.

For nearly 40 years, astronomers and physicists have assumed the
existence of this electron-positron wind based on the
properties of the pulsar and the nebula, but the wind has
never been directly detected.

"All energy from the pulsar goes to the wind, and the wind goes
to the nebula and carries the energy, so the wind is kind of the
bridge between the nebula and the pulsar," Aharonian told
SPACE.com. "We've never seen the wind, but we postulate its
existence because we cannot experience anything in the Crab
nebula without that wind. It came from a theory, but we fully
believe that – because we don't have other ideas of how these
energies can be transferred from the pulsar to the nebula."

The focus on the pulsar itself, Aharonian said, had been
understandable: "The period of the pulsed emission is exactly the
same as the rotational period of the pulsar, so the conclusion to
some extent seemed obvious: If you see pulsed emission with
exactly the same period, then you expect that this emission comes
from the pulsar."

But as the wind flows from the pulsar to the nebula, the
particles are
interacting with photons from the pulsar, which creates this
effect, he added. Photons are the smallest and most basic unit of
light.

"Photons are everywhere, but the density is not sufficient to
produce observable and
detectable gamma-rays," Aharonian said.

The researchers studied the behavior of the wind along its path
and noted that it is dominated by electromagnetic energy at the
start, but the energy becomes more kinetic "somehow, somewhere"
as the wind moves away from the pulsar, Aharonian said. "When
this conversion takes place, the wind gets accelerated to much
higher energies."

The scientists were also able to narrow down the likely area
where the energy is converted.

"The wind cannot be very close to the pulsar, otherwise the
gamma-ray emissions will be higher," Aharonian said. "But the
wind cannot be much farther from the pulsar, because the target
photons will be dulled with distance. So there should be a place
of compromise."

The researchers calculated a narrow zone where the wind probably
accelerates, but Aharonian said future studies will be needed to
better characterize the behavior of the wind and the resulting
gamma-ray energy.

"If we know that the radiation comes from this wind, which was
before expected to be invisible, and we know where the wind is
produced, now future theories should explain why wind accelerates
when it does," Aharonian said.

You can follow SPACE.com staff writer Denise Chow on
Twitter@denisechow.
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